With thick red-velvet milkshakes, classic cheeseburgers and hot dogs, the retrostyle diner is making a big splash uptown, but it needs to improve its spotty service. Photo:

Red-velvet milkshakes (
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With thick red-velvet milkshakes (inset), classic cheeseburgers and hot dogs, the retrostyle diner is making a big splash uptown, but it needs to improve its spotty service. (N.Y.Post Brian Zak (2))

Harlem Shake’s eponymously named fountain treat is Mayor Bloomberg’s death-by-diabetes nightmare — a half-liquid that’s like red-velvet cake run through a blender with peanut butter and ice cream.

Even the small size ($4.50, $6.25 for a large) must contain 1 million fat-and-sugar calories. But this is the new Harlem, so the ice cream in the shake is fancy Blue Marble brand; the milk’s organic; cupcake chunks by Tonnie’s Minis; and the salted caramel house-made.

The menu also informs us that beef burgers are made with Pat LaFrieda sirloin — not what past generations would expect at Lenox Avenue and West 124th Street.

Harlem Shake has enjoyed months of easy publicity over its name, which the lucky owners picked before the unrelated dance hit went viral on YouTube. Luckier still, most of the food’s dandy for what it is: fun American diner grub à la Shake Shack with a few Southern accents, but not “soul” like at Sylvia’s a few blocks north.

Two bozo bloggers next to us smugly cackled that $6.95 for a double cheeseburger was “too much for Harlem.” They must not spend much time north of 14th Street. The slickly designed,luncheonette-style cafe (open till 11 p.m.) holds down a corner of the historic neighborhood poised between the storied past and a thrilling, but pricier present.

Neighbors include traditional shops such as Touba African Clothing but also Marcus Samuelsson’s Red Rooster cafe. The next block will soon be home to Whole Foods. But to outlast the buzz, Harlem Shake must win over both newcomers and longtime locals. It needs to up its service game and get a working phone: The numbers on the check and Web site are wrong.

There’s no table service: Order at the counter and pick it up when a sex toy-like gizmo buzzes and vibrates. But things get wobbly when lines grow long.

At a hectic lunch hour, ever-smiling order-takers said I couldn’t have tomato with grilled cheese because, “We don’t have tomatoes,” even though they appear in several dishes.

The tomato-less sandwich was lame. They lost track of my order for a single hot dog. Customers grumbled, “Where’s my chili-cheese fries?”

But on a less frenzied, rainswept night, the “Harlem classic with cheese” burger — two thin sirloin patties with American cheese, pickles and the works on a potato roll — rang the bell despite the house way of griddling them well-done. “The Big Mac of Harlem,” my friend declared, but the Golden Arches never knew beef so richly flavorful.